National Instruments PXIe-4340, User Manual

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© National Instruments | 2-15
NI PXIe-4340 User Manual
Note
For control applications, it is important to consider the group delay of the data
being acquired and analyzed when calculating the control system bandwidth.
Regardless of the sample rate, the bandwidth of the system is as follows:
2 kHz Control Loop Rate Calculation Example
Figure 2-11 represents a typical control system in which you have a process to control, a DAQ
device for signal measurement and stimulus generation, and a PXI Express controller or
computer to process data and return an appropriate AO value to the process.
Figure 2-11. Typical Control System
To successfully close a 2 kHz control loop, make sure that the time difference between the time
the AI sample is acquired and the time the AO stimulus is generated is <500
μ s. Refer to
Figure 2-12.
Figure 2-12. Input and Output of a Control System with Bandwidth
≥
2 kHz
To make sure that your application can run and control a process at 2 kHz, and that the first
output is generated within the first sample period, make sure that the following conditions are
satisfied:
Transfer Time
+
Application Time
≤ 500 µs (2-1)
where:
Transfer Time
—the time it takes to transfer samples between the DAQ device and the
PXI Express controller or computer.
Application Time
—the time it takes for the PXI Express controller or computer to analyze
the acquired data and generate the AO stimulus.
bandwidth
1
TransferTime ApplicationTime AnalogGroupDelay HWTSP Latency
+ + +
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
=
Control Loop Time, 500
µ
s
PXI Expre ss Controller or Comp u ter Proce ss DAQ Device
AI Me asu rement
AO S tim u l us
Tr a n s fer + Applic a tion < 500
µ
s
2 kHz Inp u t
2 kHz O u tp u t
Time
Sa mple 0 . . . Sa mple 1
Sa mple 0 . . . Sa mple 1